Preheat the oven to 180°C (160°C fan) and line a 2lb loaf tin.
For the base, beat the butter and sugar for 3–4 minutes until creamy.
Add the flour and salt and mix to a soft, crumbly dough.
Press into the tin, prick with a fork and bake for 20–25 minutes until lightly golden.
Leave to cool.
For the caramel, add all the ingredients to a saucepan and cook over a low-medium heat, stirring continuously, until thick and golden.
Pour over the cooled base, spread evenly and chill for 1 hour until set.
Pour the melted dark chocolate over the caramel and spread evenly.
Drizzle lines of white chocolate across the top.
While still soft, drag a skewer through the chocolate in alternating directions to create a feathered pattern.
Chill for 10 minutes until firm, then remove from the tin and cut into bite-sized pieces.
